This invention relates generally to CNC machine tools and more particularly to automated error compensation in such machines.
Rotary work piece machine tools, such as horizontal lathes and vertical turning lathes, are commonly used to machine and produce parts in an efficient and repeatable manner. In machining such parts, it is often necessary to measure certain machined dimensions to assure the accuracy of the part's dimensions. Traditionally, turned work piece diameters have been measured using manual gauges such as micrometers, vernier gauges and other types of bridge gauges. These gauges are manually employed by the machine operator and are positioned by feel and/or by observation of an indicator to establish the on center condition. However, these manual operations are time consuming, use costly gauges and are dependent on the individual operator's skill level. This serves to limit the capacity of the machine tool and drives up the cost of producing the parts. Furthermore, all such manual operations are prone to introducing errors into the machining process.
To avoid these drawbacks, improved approaches to machine gauging have been sought. One such approach is known as "on machine probing." On machine probing employs a sensor, such as a touch trigger type displacement probe, to probe machined surfaces of the work piece. The probe is mounted into the machine tool's tool holding structure and uses its axis and positioning feedback system to determine various work piece dimensions. However, if the centerline of the probe does not crossover the work piece's centerline axis of rotation as it moves along its path of travel, then the probing will not measure the true diameter, but a non-diametrical chord of the work piece. The difference between the true diameter and a non-diametrical chord is referred to herein as the "probe chord error."
To contain the probe chord error within acceptable limits, the path of travel of the probe's centerline needs to be set to a specific tolerance and frequently monitored to maintain the tolerance. This can be a difficult and time consuming task. One approach for automating the monitoring aspect is described in U.S. Pat. No. 5,373,222 issued December 13, 1994 to R. David Hemmerle et al. This patent describes a methodology for rapidly evaluating the probe's crossover error, i.e., the displacement of the probe's path of travel from the work piece's centerline axis of rotation. However, once the crossover error is determined, the probe's path of travel alignment has to be manually adjusted so that the crossover error is within tolerance. To minimize measurement accuracy loss due to crossover error, a tight tolerance and frequent evaluations and adjustments are required. This requires special vigilance and skill in setting, and consumes maintenance and machine time.
Accordingly, there is a need for a method of automatically compensating for probe chord error that does not require frequent probe adjustments.